1. Field of the Invention
The present invention relates to a neutral hold device for a hydrostatic continuously variable speed change device.
2. Description of the Related Art
In a neutral hold device for a hydrostatic continuously variable speed change device such as the one mentioned above, an operating arm is linked to a speed change pedal or other speed shifter via a linking mechanism. In a neutral hold device, the operating arm rotates integrally with a trunnion shaft while pushing out a cam follower (roller) from the bottom of the arm cam through the operation of the speed shifter against the urging of a spring. The speed of the hydrostatic continuously variable speed change device is thereby changed according to the operation direction and amount of operation from the neutral position of the speed shifter, and the arm is pivoted by the urging of the spring in conjunction with the withdrawal of operation of the speed shifter so that the cam follower faces the bottom of the cam. The operating arm rotates integrally with the trunnion shaft toward the neutral position in conjunction with the pivoting action, whereby the trunnion shaft is returned to the neutral position, and the hydrostatic continuously variable speed change device is placed in the neutral state (see JP 2001-277888, for example).
In the configuration described above, when the urging force of the spring decreases due to changes that occur over time, the trunnion shaft returns near the neutral position through the urging of the spring when operation of the speed shifter is withdrawn, but the trunnion shaft may no longer return to the neutral position. The trunnion shaft also ceases to return to the neutral position in such cases as when the viscosity of the hydraulic oil in the hydrostatic continuously variable speed change device is increased by a decrease in the ambient temperature.
In other words, reduction of the spring force due to changes that occur over time, increased viscosity of the hydraulic oil due to decreased ambient temperature, and other effects can lead to drawbacks whereby the neutral state of the hydrostatic continuously variable speed change device is no longer obtained. Increasing the urging force of the spring is one method of preventing this drawback, but the speed change operation of the hydrostatic continuously variable speed change device against the urging of the spring becomes too stiff in this case. Drawbacks therefore occur in that the operability of the hydrostatic continuously variable speed change device is reduced.